Significant growth in both high-frequency wired and wireless markets has introduced new opportunities where compound semiconductors such as SiGe have unique advantages over bulk complementary metal oxide semiconductor (CMOS) technology. With the rapid advancement of epitaxial-layer pseudomorphic SiGe deposition processes, epitaxial-base SiGe heterojunction bipolar transistors have been integrated with mainstream advanced CMOS development for wide market acceptance, providing the advantages of SiGe technology for analog and RF circuitry while maintaining the full utilization of the advanced CMOS technology base for digital logic circuitry.
A typical prior art SiGe heterojunction bipolar transistor is shown, for example, in FIG. 1. Specifically, the SiGe heterojunction bipolar transistor shown in FIG. 1 comprises semiconductor substrate 10 of a first conductivity type having sub-collector 14 and collector 16 formed therein. Isolation regions 12, which are also present in the substrate, define the outer boundaries of the bipolar transistor. The bipolar transistor of FIG. 1 further includes SiGe layer 20 formed on a surface of substrate 10 as well as isolation regions 12. The SiGe layer includes polycrystalline Si regions 24 that are formed over the isolation regions and SiGe base region 22 that is formed over the collector and subcollector regions. The prior art bipolar transistor also includes patterned insulator layer 26 formed on the base region and emitter 28 formed on the patterned insulator layer as well as a surface of SiGe base region 22. Silicide regions 30 are also present in the structure shown in FIG. 1.
A major problem with the prior art SiGe heterojunction bipolar transistors of the type illustrated in FIG. 1 is that the SiGe bipolar yield is significantly reduced because of the presence of shorts which are introduced into the structure during the silicide process. The shorts are caused by the presence of silicide bridges that exist in the structure. As such, a 20–30% yield loss is typically associated with prior art SiGe heterojunction bipolar transistors. The SiGe bipolar yield loss is more pronounced when cobalt disilicide regions are formed in the structure.
In view of the above mentioned problem with prior art heterojunction bipolar transistors, there is still a continued need for developing a new and improved method which is capable of fabricating a heterojunction bipolar transistor in which the SiGe bipolar yield loss due to silicide shorts has been substantially eliminated.